According to the zoo's head of conservation, Amy Plowman, the bananas' high sugar content could cause the monkeys to develop diabetes or similar conditions.

"It can also cause gastrointestinal problems as their stomachs are mostly adapted to eating fibrous foods with very low digestibility," she said.

Zoos are getting smarter in the way they feed their captive animals.

Rather than throw in ready chopped and prepared food, many now try to make the animals "work" for their calories by searching for scattered foodstuffs or doing puzzles to release treats.

This is just the next step, Plowman said.

Knowling said there was no obvious sign that the monkeys were missing the fruit. In fact, they could well be happier without the extra sugar.

"Smaller monkeys such as tamarins and marmosets are highly strung animals and live in tight-knit social groups which can be quite aggressive at times," said Matthew Webb, senior head keeper of mammals.

"Reducing the sugar in their diets has calmed them down and made their groups more settled."